1. Field of the Invention
The invention relates generally to cross-linked polyethylene (PEX) pipe of the type typically used in residential and commercial applications and, more particularly, to an improved braided PEX pipe having increased burst and temperature ratings that render the braided PEX pipe particularly suited for use in high pressure/high temperature applications in the residential and commercial sectors, such as National Fire Protection Association (“NFPA”) 13R fire sprinkler applications.
2. Related Art
PEX pipe is commonly manufactured from high density polyethylene (“HDPE”) that experiences a change in molecular structure whereby its polymer chains are chemically linked (i.e. cross-linked) with one another to form a three-dimensional network. The most common catalysts for causing such a change in molecular structure are: a peroxide mixed with HDPE before extrusion (“Engel Method”), a silane-based cross-linking agent incorporated into HDPE resin either prior to, or during, extrusion, or a high radiation electron beam irradiated onto an extruded HDPE pipe.
Product manufactured using the Engel Method is classified as PEX-A, PEX-Xa, or PEXa. The Engel (“peroxide”) Method performs “hot cross-linking” because the cross-linking occurs during passage of melted and peroxide-infused HDPE under high pressure through a heated die. Although often more expensive than the electron irradiation and silane-based methods, the Engel Method generally provides more consistent and uniform cross-linking (85% or more) than the other methods. As a result, the Engel Method allows precise control over the degree of cross-linking. The resultant cross-link bonds are carbon-to-carbon based.
Product manufactured using the silane-based method (or “moisture cure method”) is classified as PEX-B, PEX-Xb, or PEXb. A two-step (Sioplas) or one-step (Monosil) reaction can be implemented to cross link HDPE using silane. In the Sioplas method, a silane, such as vinylsilane, for example, can be grafted upon HDPE resin prior to extrusion. In the Monosil method, a silane may be blended with the HDPE resin during extrusion. Regardless of the silane method used, cross-linking occurs in a secondary post-extrusion process that is accelerated with heat and moisture, i.e., when the resulting extruded pipe is exposed to a hot water or steam bath. Unlike the Engel or electron irradiation methods, the resultant cross-link bonds are not between carbon atoms, but are oxygen-silicon-oxygen bridges, resulting in less uniform (65% to 70%) levels of cross-linking.
Product manufactured using the electron irradiation method is classified as PEX-C, PEX-Xc or PEXc. The electron irradiation method is a physical process, rather than a chemical process, that achieves “cold” cross-linking. As a result, this method of cross linking can be used on already extruded HDPE pipe without any special chemical additives. The extruded HDPE pipe is routed under a high radiation electron beam, which releases hydrogen atoms from adjoining polymer chains that permits the cross-linking of the “open” carbon sites. This method yields less-uniform (70-75%) cross-linking than the Engel Method. Moreover, if the irradiation process is not properly controlled, the PEX pipe may become stiffer and discolored as a result of oxidation.
Product manufactured using one of these three processes may also include an oxygen barrier. One example is RAUPEX O2 Barrier Pipe manufactured by the assignee of this application, REHAU Inc. of Leesburg, Va., which has a co-extruded oxygen diffusion barrier that meets or exceeds industry standards.
Another example is PEX-AL-PEX pipes or PEX/Aluminum/PEX pipes that are manufactured to include a layer of aluminum between two opposing layers of PEX. The aluminum layer acts as an oxygen barrier and prevents oxygen from permeating through the PEX and corroding metal components that may be attached to the PEX pipe. A disadvantage of PEX-AL-PEX pipes, however is that repeated expansion and contraction cycles may degrade the outer PEX coating and result in corrosive galvanic contact between the aluminum and a brass compression fitting attached to the pipe.
Additionally, PEX pipe may include chlorine resistant and/or ultraviolet light resistant properties. For example RAUPEX Non-Barrier Pipe (plumbing only) meets industry standards for chlorine resistance. Additionally, RAUPEX Red/Blue UV Shield Pipe (plumbing only) has a co-extruded polyethylene shield that provides additional protection for the pipe against ultraviolet light. Both the non-barrier pipe and the UV shield pipe are made of high quality PEX manufactured using the Engel Method (e.g., “high pressure peroxide method”).
PEX pipe is predominantly used in low pressure hydronic radiant heating systems and also in low pressure residential and commercial plumbing systems and in some (low temperature/low pressure) fire protection systems. It is also used in the offshore oil sector, in natural gas applications, and in the transportation of slurries or sewage. Presently, PEX pipe has temperature and pressure ratings of approximately 160 psi at about 73.4° F., 100 psi at about 180° F., and 80 psi at 200° F., as well as a minimum burst rating as high as 475 psi at 73.4° F. (⅝ inch tube and larger). These ratings are for a minimum lifetime of 50 years under these sustained conditions, and these ratings can be tested in given pipes pursuant to the guidelines of ASTM International's Standard for Crosslinked Polyethylene (PEX) Tubing, F 876-04 (approved May 1, 2004).
PEX pipe, which is thermoset, has excellent rebound characteristics and will return to its original shape after being deformed. Accordingly, PEX pipe can withstand cracking caused by exposure to cold weather and high pressure fluids. It is also corrosion resistant. Conventional PEX pipe, however, has a low to moderate short term burst pressure that makes it unsuitable for high pressure applications such as multifamily residential fire protection system (e.g., NFPA 13/13R Fire Sprinkler Design Standards), oil field recovery applications, natural/propane gas, or high pressure compressed air systems.
NFPA 13 (“Standard for the Installation of Sprinkler Systems”) is the design standard typically used for fire sprinkler installations in most commercial buildings. The standard requires water flow sufficient to support not less than four fire sprinklers, and may impose additional water flow if the property includes large places for people to gather. The standard also requires installation of fire sprinklers in attics and other unoccupied spaces.
In contrast, the NFPA 13R (“Standard for the Installation of Sprinkler Systems in Residential Occupancies up to and Including Four Stories in Height”) does not require installation of fire sprinklers in unoccupied spaces and does not require a minimum flow of water to support four fire sprinklers. Thus, if the largest room can be protected with less than four sprinklers, then the water supply can be reduced to the lesser requirement, which translates to lower installation costs.
Currently, there is only one plastic pipe approved for this application, i.e., chlorinated polyvinyl chloride (CPVC), which is a rigid pipe and requires glue and solvent to make the joints. There are many issues with CPVC pipe and glue/solvent connections, including dry fitting and leaking concerns which may increase installation time and costs. CPVC is also formed of inflexible plastic, thereby increasing the costs of installation because it cannot be laid in extended lengths without joints to accommodate the structure in which it is installed. Further, CPVC's inflexibility makes it difficult to retrofit piping in older buildings or other applications where curved piping would be desirable.
Nevertheless, as discussed above, currently available PEX pipe has a short term burst pressure rating that is insufficient for use in sprinkler systems meeting the 13/13R standards, which may require short term burst ratings over 1000 psi and a long term hydrostatic strength of about 175 psi/120° F., for example. In addition to fire sprinkler systems, these higher temperature/pressure tolerances are required for a number of applications such as oil field recovery applications, natural gas, high pressure compressed air systems, and long range piping systems. Typical PEX pipe is not currently capable of achieving these higher pressure/temperature ratings. Indeed, there is no flexible, polymer piping product available in the market capable of handling the aforementioned high temperature/higher pressure applications.
Thus, there is a need for an improved, flexible PEX pipe that can be used in high pressure/high temperature applications including, but not limited to, those mentioned above, like fire sprinkler systems in commercial and high occupancy residential buildings. Moreover, there is a need for a flexible, higher pressure pipe that avoids the installation issues with rigid CPVC pipes, and can reduce installation time and costs by minimizing the amount of fittings and/or provide a more secure system minimizing the potential for leaks and water damage.